The present invention relates to protection systems for preventing overheating of heat pipe airheaters and, in particular, to systems having a dual passage with heat pipes mounted in a wall separating the flow paths of the dual passage.
A heat pipe is a device that achieves a high thermal conductance by taking advantage of the large latent heat of vaporization/condensation of a working fluid. An internal working fluid can absorb heat at the evaporator end of the pipe by vaporizing. Vapor thus generated moves to the condenser end where the fluid surrenders heat and condenses. A thermo-syphon heat pipe uses gravity, buoyancy and vapor pressure forces to transport the phases of the working fluid. This type of heat pipe normally requires the evaporator end to be lower than the condenser end. The working medium cycles because the lighter vapor is buoyed and the condensed liquid falls. Alternatively, apparatus can be included in the heat pipe to transport the working medium by capillary action using gauze, wire mesh or other suitable materials.
The heat pipe can be used in various applications, such as recovering waste heat from a plant to heat air for various purposes including heating a living space. The heat pipes can also be used in steam condensers wherein cooling air is blown over the condenser end of the heat pipe. In flue gas reheaters, heat can be extracted by a heat pipe from flue gas prior to entering a scrubber module. The heat is returned to the flue gas exiting the scrubber to provide sufficient buoyancy as the gas enters the atmosphere. For airheater applications, heat is transferred from the exhaust gas of the furnace or boiler to the incoming combustion air.
Heat pipe airheaters utilize synthetic fluids and sometimes corrosion inhibitors that disassociate or break down at elevated temperatures. Thus, if the hot gas flowing over the evaporator end exceeds design limits or if the gas to air flow rate is substantially greater than design, overheating, degraded performance and premature failure can occur.
In pulverized-coal-fired boilers, oil or other fossil fuel may be used to start the boiler before the furnace is sufficiently hot to accept pulverized coal. Combustion air or primary air should not pass through the coal pulverizing units during this start up phase because of the danger of explosion from the dispersing of coal dust in the pulverizer. Since primary air does not pass through the airheater but hot flue gas does, the airheater can overheat.
Furthermore, even when the boiler is operating in its normal mode, the airheater can become unbalanced and overheat. During normal operation on a pulverized coal unit, convection surfaces may become fouled. The gas temperature entering the heat exchanger may therefore become substantially greater than designed. For circulating fluidized bed boilers, high gas temperature can occur if high particle carry over occurs at the cyclone. Any of these events may lead to deterioration of the heat transfer fluid in the evaporating end of the heat pipe in an airheater.
Heat pipe air heaters as used on fossil-fired utility boilers for exchanging heat between flue gas and combustion air, are designed to have minimum leakage from flue gas to combustion air path. This design criteria is important when there is a large pressure differential, as occurs between the primary air and gas on pulverized-coal-fired units and on fluidized bed boilers. Efficiency considerations make it normally desirable to reduce air leakage across the airheater to reduce the power required for the primary air blower; as well as reducing precipitator and baghouse requirements.
In U.S. Pat.No. 4,015,932 a combustion air preheater has a temperature sensor at the downstream end of the air passage. The sensed temperature can operate a venting valve to increase the air flow through the air passage thereby affecting the air velocity and temperature. This reference, however, does not discuss cross feeding air from one side of a heat exchanger to the other.
U.S. Pat. No. 4,449,569 shows an air preheater for a furnace in which temperature is sensed in the air passage and used to regulate an induced draft fan. Again, this reference does not show cross feeding air from one side of an exchanger to the other. See also U.S. Pat. Nos. 4,029,465; 4,040,477; 4,589,844; and 4,784,069.
Accordingly, there is a need for a system for protecting airheaters in a way that is simple and effective.